Examination of charge order in mixed valence oxide LuFe₂O₄ by mössbauer quadrupole effect

The Fe²⁺=Fe³⁺ charge order (CO) in LuFe₂O₄ has been examined by Mössbauer quadrupole effect. The stoichiometric single crystal of LuFe₂O₄ was used to measure the paramagnetic spectrum at 282 K. The spectrum was analyzed by assuming the CO monoclinic structure (space group C2=m) with four inequivalent Fe sites, among which two are occupied by Fe³⁺ and other two are occupied by Fe²⁺. The obtained quadrupole parameters, quadrupole splitting QS and the intensity ratio of the two absorption lines I(±3=2)=I(±1=2), coincide well with those calculated from the crystal structure. On the other hand, the parameters obtained by assuming other two crystal structures previously proposed, the hexagonal structure with a single Fe site (R3 m) and the monoclinic structure with a single Fe site (C2=m), are found to be inconsistent with the calculated ones. These results indicate that the CO monoclinic structure is most probable among the structural models proposed so far from the viewpoint of the Mössbauer quadrupole effect.